Postnikov factorizations at infinity

We have developed Postnikov sections for Brown–Grossman homotopy groups and for Steenrod homotopy groups in the category of exterior spaces, which is an extension of the proper category. The homotopy fibre of a fibration in the factorization associated with Brown–Grossman groups is an Eilenberg–Mac Lane exterior space for this type of groups and it has two non-trivial consecutive Steenrod homotopy groups. For a space which is first countable at infinity, one of these groups is given by the inverse limit of the homotopy groups of the neighbourhoods at infinity, the other group is isomorphic to the first derived of the inverse limit of this system of groups. In the factorization associated with Steenrod groups the homotopy fibre is an Eilenberg–Mac Lane exterior space for this type of groups and it has two non-trivial consecutive Brown–Grossman homotopy groups. We also obtain a mix factorization containing both kinds of previous factorizations and having homotopy fibres which are Eilenberg–Mac Lane exterior spaces for both kinds of groups.Given a compact metric space embedded in the Hilbert cube, its open neighbourhoods provide the Hilbert cube the structure of an exterior space and the homotopy fibres of the factorizations above are Eilenberg–Mac Lane exterior spaces with respect to inward (or approaching) Quigley groups.     

The strength of Mac Lane set theory

Saunders Mac Lane has drawn attention many times, particularly in his book Mathematics: Form and Function, to the system of set theory of which the axioms are Extensionality, Null Set, Pairing, Union, Infinity, Power Set, Restricted Separation, Foundation, and Choice, to which system, afforced by the principle, , of Transitive Containment, we shall refer as . His system is naturally related to systems derived from topos-theoretic notions concerning the category of sets, and is, as Mac Lane emphasises, one that is adequate for much of mathematics. In this paper we show that the consistency strength of Mac Lane's system is not increased by adding the axioms of Kripke–Platek set theory and even the Axiom of Constructibility to Mac Lane's axioms; our method requires a close study of Axiom H, which was proposed by Mitchell; we digress to apply these methods to subsystems of Zermelo set theory , and obtain an apparently new proof that is not finitely axiomatisable; we study Friedman's strengthening of , and the Forster–Kaye subsystem of , and use forcing over ill-founded models and forcing to establish independence results concerning and ; we show, again using ill-founded models, that proves the consistency of ; turning to systems that are type-theoretic in spirit or in fact, we show by arguments of Coret and Boffa that proves a weak form of Stratified Collection, and that is a conservative extension of for stratified sentences, from which we deduce that proves a strong stratified version of ; we analyse the known equiconsistency of with the simple theory of types and give Lake's proof that an instance of Mathematical Induction is unprovable in Mac Lane's system; we study a simple set theoretic assertion—namely that there exists an infinite set of infinite sets, no two of which have the same cardinal—and use it to establish the failure of the full schema of Stratified Collection in ; and we determine the point of failure of various other schemata in . The paper closes with some philosophical remarks.     

The Steenrod Algebra and Theories Associated to Hopf Algebras

We show that the homotopy category of products of Z/p-Eilenberg–Mac Lane spaces is an -algebra which algebraically is determined by the Steenrod algebra considered as a Hopf algebra with unstable structure.     

A cohomological characterization of Alexander schemes

Vistoli defined Alexander schemes in [19], which behave like smooth varieties from the viewpoint of intersection theory with Q-coefficients. In this paper, we will affirmatively answer Vistoli’s conjecture that Alexander property is Zariski local. The main tool is the abelian category of bivariant sheaves, and we will spend most of our time for proving basic properties of this category. We show that a scheme is Alexander if and only if all the first cohomology groups of bivariant sheaves vanish, which is an analogy of Serre’s theorem, which says that a scheme is affine if and only if all the first cohomology groups of quasi-coherent sheaves vanish. Serre’s theorem implies that the union of affine closed subschemes is again affine. Mimicking the proof line by line, we will prove that the union of Alexander open subschemes is again Alexander. Oblatum 1-XII-1997 & 14-XII-1998 / Published online: 10 May 1999     

Local normal forms of smooth weakly hyperbolic integrable systems

In the smooth (C∞) category, a completely integrable system near a nondegenerate singularity is geometrically linearizable if the action generated by the vector fields is weakly hyperbolic. This proves partially a conjecture of Nguyen Tien Zung [11]. The main tool used in the proof is a theorem of Marc Chaperon [3] and the slight hypothesis of weak hyperbolicity is generic when all the eigenvalues of the differentials of the vector fields at the non-degenerate singularity are real.     

A simple permutoassociahedron

In the early 1990s, a family of combinatorial CW-complexes named permutoassociahedra was introduced by Kapranov, and it was realised by Reiner and Ziegler as a family of convex polytopes. The polytopes in this family are “hybrids” of permutohedra and associahedra. Since permutohedra and associahedra are simple, it is natural to search for a family of simple permutoassociahedra, which is still adequate for a topological proof of Mac Lane’s coherence. This paper presents such a family.     

A coherence theorem for canonical morphisms in cartesian closed categories

In this paper there is proved a coherence theorem in proof-theoretic formulation: all derivations of a balanced sequent are equivalent. (A sequent is called balanced if each variable appears in it no more than twice.) Canonical morphisms in a Cartesian closed category are morphisms which can be obtained from those explicitly mentioned in the definition of a Cartesian closed category (i.e., the left and right projectionsl: A x B A and r:A x B B, :A x hom (A, B) B, etc.) with the help of composition of functors x, hom and the operation +. Let the objects A and B be constructed from the objects C₁,..., C_n with the help of the functors x and hom. Then, generally speaking, not all canonical morphisms from A to B will be equal. For example, if A is C₁ x C₁, and B is C₁, then the left and right projections are different morphisms. The coherence theorem asserts that if one does not make superfluous identifications of objects, then all canonical morphisms from A to B will be equal, i.e., all diagrams of canonical morphisms beginning in A and ending in B will commute. There is a familiar translation of certain concepts of the theory of categories into the language of proof theory, under which to objects correspond formulas, and the functors x and hom are interpreted as the connectives & and . Under this translation, to canonical morphisms from A to B correspond derivations in the (&, )-fragment of the intuitionistic prepositional calculus of the sequent A B. Morphisms are equal if and only if the derivations corresponding to them are equivalent, i.e., certain of their normal forms coincide, or, what is the same thing, their deductive terms are equivalent. The theorem proved in this paper is equivalent with the coherence theorem in the algebraic formulation. There are given two proofs of this theorem, obtained independently by the authors, in one of which there are considered natural derivations and the apparatus of deductive terms is used, and the other is based on reduction of the depth of formulas preserving equivalence of derivations, specialization of forms of inference in Gentzen L-systems, and analysis of links in sequences.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 88, pp. 3–29, 1979.     

Finiteness conditions in the stable module category

We study groups whose cohomology functors commute with filtered colimits in high dimensions. We relate this condition to the existence of projective resolutions which exhibit some finiteness properties in high dimensions, and to the existence of Eilenberg–Mac Lane spaces with finitely many n-cells for all sufficiently large n. To that end, we determine the structure of completely finitary Gorenstein projective modules over group rings. The methods are inspired by representation theory and make use of the stable module category, in which morphisms are defined through complete cohomology. In order to carry out these methods, we need to restrict ourselves to certain classes of hierarchically decomposable groups.     

Proof of semialgebraic covering mapping cylinder conjecture with semialgebraic covering homotopy theorem

In this paper we prove the semialgebraic version of Palais' covering homotopy theorem, and use this to prove Bredon's covering mapping cylinder conjecture positively in the semialgebraic category. Bredon's conjecture was originally stated in the topological category, and a topological version of our semialgebraic proof of the conjecture answers the original topological conjecture for topological G-spaces over “simplicial” mapping cylinders.     

Proof of grünbaum's conjecture on common transversals for translates

In 1958 B. Grünbaum made a conjecture concerning families of disjoint translates of a compact convex set in the plane: if such a family consists of at least five sets, and if any five of these sets are met by a common line, then some line meets all sets of the family. This paper gives a proof of the conjecture.     

Braided premonoidal Mac Lane coherence

Given a category with a bifunctor and natural isomorphisms for associativity, commutativity and left and right identity we do not assume that extra constraining diagrams hold. We introduce groupoids of coupling trees to describe versions of coherence that are weaker than the usual notion of Mac Lane's monoidal coherence.     

Permanence of Metric Sparsification Property under Finite Decomposition Complexity

The notions of metric sparsification property and finite decomposition com- plexity are recently introduced in metric geometry to study the coarse Novikov conjecture and the stable Borel conjecture. In this paper, it is proved that a metric space X has finite decomposition complexity with respect to metric sparsification property if and only if X itself has metric sparsification property. As a consequence, the authors obtain an alterna- tive proof of a very recent result by Guentner, Tessera and Yu that all countable linear groups have the metric sparsification property and hence the operator norm localization property.     

The Martin Boundary of the Young-Fibonacci Lattice

In this paper we find the Martin boundary for the Young-Fibonacci lattice YF. Along with the lattice of Young diagrams, this is the most interesting example of a differential partially ordered set. The Martin boundary construction provides an explicit Poisson-type integral representation of non-negative harmonic functions on YF. The latter are in a canonical correspondence with a set of traces on the locally semisimple Okada algebra. The set is known to contain all the indecomposable traces. Presumably, all of the traces in the set are indecomposable, though we have no proof of this conjecture. Using an explicit product formula for Okada characters, we derive precise regularity conditions under which a sequence of characters of finite-dimensional Okada algebras converges.     

Combinatorial analysis of proofs in projective and affine geometry

The axioms of projective and affine plane geometry are turned into rules of proof by which formal derivations are constructed. The rules act only on atomic formulas. It is shown that proof search for the derivability of atomic cases from atomic assumptions by these rules terminates (i.e., solves the word problem). This decision method is based on the central result of the combinatorial analysis of derivations by the geometric rules: The geometric objects that occur in derivations by the rules can be restricted to those known from the assumptions and cases. This “subterm property” is proved by permuting suitably the order of application of the geometric rules. As an example of the decision method, it is shown that there cannot exist a derivation of Euclid’s fifth postulate if the rule that corresponds to the uniqueness of the parallel line construction is taken away from the system of plane affine geometry.     

Brown–Peterson Cohomology from Morava K-Theory

We give some structure to the Brown–Peterson cohomology (or its p-completion) of a wide class of spaces. The class of spaces are those with Morava K-theory even-dimensional. We can say that the Brown–Peterson cohomology is even-dimensional (concentrated in even degrees) and is flat as a BP*-module for the category of finitely presented BP*(BP)-modules. At first glance this would seem to be a very restricted class of spaces but the world abounds with naturally occurring examples: Eilenberg-Mac Lane spaces, loops of finite Postnikov systems, classifying spaces of most finite groups whose Morava K-theory is known (including the symmetric groups), QS²ⁿ, BO(n), MO(n), BO, Im J, etc. We finish with an explicit algebraic construction of the Brown–Peterson cohomology of a product of Eilenberg–Mac Lane spaces and a general Künneth isomorphism applicable to our situation.     

An elementary proof for the nine missing particles of the standard model

Several authors have recently argued that a “complete” standard model should include nine more elementary particles besides the 60 already believed to be experimentally confirmed. The present short note gives an elementary and convincing proof for the correctness of this conjecture.     

Cyclic orders: Equivalence and duality

Cyclic orders of graphs and their equivalence have been promoted by Bessy and Thomassé’s recent proof of Gallai’s conjecture. We explore this notion further: we prove that two cyclic orders are equivalent if and only if the winding number of every circuit is the same in the two. The proof is short and provides a good characterization and a polynomial algorithm for deciding whether two orders are equivalent. We then derive short proofs of Gallai’s conjecture and a theorem “polar to” the main result of Bessy and Thomassé, using the duality theorem of linear programming, total unimodularity, and the new result on the equivalence of cyclic orders.     

Categories of projective spaces

Starting with an abelian category , a natural construction produces a category such that, when is an abelian category of vector spaces, is the corresponding category of projective spaces. The process of forming the category destroys abelianess, but not completely, and the precise measure of what remains of it gives the possibility to reconstruct out from , and allows to characterize categories of the form , for an abelian (projective categories). The characterization is given in terms of the notion of “Puppe exact category” and of an appropriate notion of “weak biproducts”. The proof of the characterization theorem relies on the theory of “additive relations”.